MRI is an imaging method which excites nuclear spin of an object set in a static magnetic field with a RF (radio frequency) signal having the Larmor frequency magnetically and reconstructs an image based on MR (magnetic resonance) signals generated due to the excitation.
In the field of magnetic resonance imaging, there is an imaging method called echo planar imaging (EPI). EPI is one of the high speed imaging methods in MRI. EPI is an imaging method for performing a scan which inverts a gradient magnetic field with high speed continuously after a single nuclear magnetic excitation so as to generate echoes continuously. More specifically, in EPI, all the data necessary for image reconstruction are acquired by generating continuous gradient echoes by steps of phase encodes (PE) before a magnetization in a x-y plane attenuates and disappears by transverse relaxation (T2 relaxation) after applying an excitation pulse (FLIP PULSE). EPI includes SE EPI with using a spin echo (SE) method to acquire spin echo signals generated after an excitation pulse and a refocus pulse (FLOP PULSE), FE EPI with using a field echo (FE) method to acquire echo signals generated after applying an excitation pulse and FFE EPI using a FFE (Fast FE) method. While using EPI to generate data for a single image with combined echo train data obtained by applying an excitation pulse plural times is called multi shot EPI; EPI to reconstruct an image by applying only a single excitation pulse is called single shot (SS) EPI. In addition, there is EPI referred to as Hybrid EPI.
Further, as an applied technology in EPI, DWI (diffusion weighted imaging) is known. DWI is an imaging method for enhancing phase shifts due to motions of imaging targets by applying gradient magnetic fields having strong intensities referred to as MPG (motion probing gradient) pulses to acquire images in which a diffusion effect in the imaging targets is enhanced. Generally, parameter images such as ADC (Apparent Diffusion Coefficient) images and/or isotropic DWIs (diffusion weighted images) are generated for diagnosis based on DWIs acquired by changing an application direction of MPG pulses and a reference image acquired by applying no MPG pulse.
Furthermore, DTI (diffusion tensor imaging) is known as an application in DWI. Generally, DTI is an imaging method for acquiring DWIs by changing an application direction of MPG pulses to image an imaging part mathematically using a tensor analysis based on the DWIs.
In EPI used for DWI and DTI, eddy currents are induced due to MPG pulses having strong intensities. Accordingly, a strain sometimes occurs in an image due to an influence of nonuniform magnetic fields by eddy currents. The strain in an image depends on application timings, intensities and directions of MPG pulses.
Therefore, positional shifts due to image strains according to conditions of MPG pulses may occur between a reference image acquired without applying any MPG pulse and a DWI as well as between DWIs acquired with changing application directions and intensities of MPG pulses. Generating ADC images based on images having such positional shifts causes artifacts and leads to reduced resolution. Alternatively, generating isotropic DWIs based on images having strains results in blurry images. In DTI, mutually different strains are also generated in images acquired with changing application directions of MPG pulses. Consequently, it may become difficult to perform a precise tensor analysis.
Under such background, various techniques for correcting a strain of an image due to MPG pulses in an image space are devised.
In DWI and DTI, it is an object to correct a strain in an image more satisfactorily.
It is an object of the present invention to provide a magnetic resonance imaging apparatus and a magnetic resonance imaging method which can obtain a DWI or a DTI of which strain is corrected more satisfactorily.